A common problem with battery powered appliances., tools or toys that operate on rechargeable battery packs is that the battery pack tends to reach a discharged condition when the device is in use. If the battery is internal to the appliance, use of the appliance must be discontinued while the battery is recharged via a conventional AC/DC battery charger. In the case of a battery powered appliance such as a home video camera, a delay in operational time often results in the irreversible loss of opportunity. This problem has been addressed by the manufacturers of battery powered appliances, such as garden tools, in some instances, by supplying a removable battery pack (U.S. Pat. Nos. 3,757,194 and 4,084,123). In this way the discharged battery pack may be replaced with a fully charged battery pack enabling the device to be operated for an additional period of time.
There are still several problems associated with the use of a secondary battery pack. The first problem relates to the significant expense associated with the purchase of the second battery pack. An additional problem is that the second battery pack is, in many instances, rarely utilized because it can only be used with the specific appliance for which it was purchased. It is for the lack of use that a second battery pack is rarely purchased. Another problem associated with a second rechargeable battery pack relates to the fact that, the second battery pack must have been thoroughly recharged within a suitable period of time, because of the tendency of rechargeable batteries to "self-discharge" during storage. For example, a fully recharged battery pack of a nickel-cadmium design, will lose approximately 1% of its nominal charge per day if stored at room temperature, ie 77 degrees Fahrenheit.
The final problem occurs when indeed the second battery pack is found to be fully charged, ready for service, and is used to operate the appliance. Ultimately, the second battery pack becomes fully discharged and the entire discharged battery pack scenario reoccurs. Accordingly, there is a need for a cost effective, portable battery charging device. This charging device must be relatively insensitive to long periods of storage, handle multiple charge/recharge cycles and possibly enable the appliance to be operated somewhat indefinitely, without the need for major delays for battery recharging.
In the past, the toy industry has addressed the issue of the charging of self-contained batteries without the need for an AC power source. In U.S. Pat. No. 3,629,680 a portable battery charger is described for a nickel-cadmium battery powered toy car. The portable charger in the form of an automotive service station gasoline pump, contains a "charging battery" and a charger cable which resembles the fuel pump nozzle. To operate, the pump nozzle is inserted into the "gas tank" of the toy car and a button depressed to release the "fuel". In this manner, the rechargeable battery of the toy car is recharged using an assembly of batteries with no need of an AC charger. The disadvantage of the "fuel pump" charger is that it requires that the toy vehicle remain completely immobile until the batteries of the toy vehicle are sufficiently recharged.
The concept of utilizing batteries to recharge other batteries has also been employed in other aspects of the battery industry. In U.S. Pat. No. 4,692,680 a series of dry cells in a package is effective as a portable boost charging device for automotive lead-acid batteries and U.S. Pat. No. 4,875,820 describes a device that uses sealed lead-acid batteries in an electronic package to recharge an automotive starting battery. This prior work shows that charge may be effectively transferred from one battery to another, but in these examples, is limited to use with electronic circuitry as a short term power boost. Finally, U.S. Pat. No. 4,670,700 indicates that nickel-cadmium batteries have been shown to be rechargeable with the use of an automotive lead acid battery. This particular technology would not be practical for use with battery powered appliances, tools or toys, due to the delay in operational time required for the recharging process.